Venus is similar in size and chemical makeup when compared with Earth—and the pair formed about the same time, more than four billion years ago. But that is apparently where the similarities end. According to a year's worth of data sent back from the European Space Agency's Venus Express orbiter launched in November 2005, the second planet from the sun is nothing like Earth—from its torrid surface to the upper reaches of its acid-laced atmosphere. The bottom line: just be glad you live here.

Here's some reasons why: Venus's surface temperature hovers around a sweltering 870 degrees Fahrenheit (465 degrees Celsius), its surface pressure is about 90 times that of Earth (which is akin to the pressure a kilometer, or 0.6 mile, below the ocean's surface), and there are no seasons there. The planet—Earth's closest neighbor—takes 243 days to turn on its own axis (and in the opposite direction) compared with Earth's swift 24-hour turnaround time. From the new data, scientists now know its atmosphere consists mostly of carbon dioxide, providing a glimpse of what global warming run amok may yield. Because of the extreme heat, water is only present in its atmosphere, so there are no oceans (and thus, no beaches). There are gale-force winds whipping about the planet, its smoglike clouds are composed of droplets of sulfuric acid (rather than water) and, contrary to previous belief, there is lightning on Venus. In other words, not only is it hot enough to evaporate an igloo on Venus in milliseconds, but you could also get struck by a bolt from the not-so-blue skies.

"Venus and Earth, they're really twins that were just separated at birth," Dimitry Titov, a researcher at the Max Planck Institute for Solar System Research in Katlenburg–Lindau, Germany, and co-author of several of nine papers in Nature on Venus Express's findings, said in a teleconference yesterday. "The key question is why those twins are so different."

An international team of scientists have been pouring over data streaming back from the Venus Express since April 2006 when it began surveying Venus, which at its minimum distance is around 25 million miles (40 million kilometers) from Earth.

At the planet's equator is a layer of turbulent air flow, which smoothes out at higher latitudes, notes Fred Taylor, a University of Oxford physicist and interdisciplinary scientist for the Venus Express mission. The wind speeds in the upper atmosphere are much faster than on Earth, partially due to the Venus's sluggish rotation. There is also evidence of vortexlike swirls of air that are thousands of miles wide at both of its poles, similar to those that appear over Earth's poles during their respective winter months.

Solar winds (gusts of ions from the sun's outer atmosphere capable of pulling apart molecules they encounter) suck up particles in Venus's atmosphere, untangle their atoms and spit them into space. Particulate matter in Earth's atmosphere is largely spared from the solar winds by our planet's strong magnetic field, something Venus lacks. As expected, scientists observed light, charged particles like hydrogen and helium ions leaving Venus's atmosphere. But they were surprised to discover that oxygen is also exiting. Researchers believe that water is being lost from the planet, because twice as many hydrogen as oxygen particles are leaving.

"Venus is very, very dry," says David Grinspoon, an astrobiologist at the Denver Museum of Nature & Science. All the water contained in its atmosphere, he adds, would be about slightly more than one inch (2.5 centimeters) deep if it were on the planet's surface. Analysis of the water leaving Venus's atmosphere, however, shows that many of the hydrogen ions are actually a stable isotope of the element called deuterium, which consists of a proton and a neutron (rather than just a proton) in its nucleus. "The amount of deuterium is an important clue to how much water has been lost over time," Grinspoon says. Researchers estimate that Venus has lost at least an ocean's worth of water since it formed, based on the deuterium particles being swept up by the solar wind.

"These differences are not just [due] to Venus being closer to the sun," Oxford's Taylor says. "We now know that the lack of a protective magnetic field and the differing planetary rotation rates also play a role in ensuring that many of the atmospheric processes we observe on Earth occur at a much faster rate on Venus. Our new data make it possible to construct a scenario in which Venus started out like the earth [did]—possibly including a habitable environment, billions of years ago—and then evolved to the state we see now."

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